The effect of the shell thickness and density on the magnetic properties of composite colloidal particles consisting of a hematite (alpha -Fe2O3) core and an yttrium oxide (Y2O3) layer is described. Pure iron oxide colloidal spheres show two clearly different trends of variation of their magnetic susceptibility, chi (m), with temperature. Below T-M approximate to 220 K, chi (m) shows a slight increase when the particles are heated; a sharp transition is observed at such a critical temperature, whereby chi (m) increases almost 3 times in a very narrow temperature interval, decreasing slowly afterward. This is the result of a well-known transition from perfect to imperfect antiferromagnetism (canted antiferromagnetism). Three types of core/shell particles have been prepared, and a gradual change is observed in chi (m) from that of hematite to that of pure Y2O3. Even the most efficiently covered particles still show a change in their chi (m)-T trends around T-M, and are clearly distinct from Y2O3 particles. Magnetization curves show that coating of hematite particles induces significant changes in the coercivity of the samples. The latter is always larger for composite than for core particles.